Engine Surging idle only on start up

Did you reset the ECM after replacing the MAF? Have you recently pulled codes?

Unplug the MAF & see what happens. With no MAF input, the ECM will default to "limp mode" fuel map. If it cures the surging idle, then the MAF is probably no good.

keel said:

A little background: got a 5.0 with a GT40 top end, TFS1 cam, and the emissions system deleted but everything else is stock (including the air box). Recently, the car started idling really rough and threw code 66 (MAF circuit below minimum voltage) so I replaced the MAF with the one from LMR and it fixed the idle and the CEL.

However, after replacing the MAF, the car now has a terrible surging idle but only when I initially start the car, it bounces between almost stalling and about 1500 RPM. Doesn't matter if the motor is hot or cold. I can calm it down with some throttle input, and then it goes away and drives flawlessly.

I also have a CEL that comes on but only at WOT, but then it turns off when I back off the gas pedal. (It didn't do this before replacing the MAF.)

Any ideas on what the problem could be?

Click to expand...

Youre gonna need to know when to spray the fuel from the injectors into the intake and also gonna need to modify the lil holes in your butterfly in your TB with a slightly larger hole.


Knowing when to spray the injectors is a fine art with aftermarket cams especially ones with alot more overlap than stock because you can literally throw fuel out of the exhaust or even overheat the intake valves by not taking advantage of the atomzation off of the hot valves which also cools them ..


This is gonna require a tuning chip to accomplish as youre gonna need to change idle settings and tailor your maf transfer and injector size etc....

Most try to bandaid the issue by advancing the distributor and turning the throttlestop screw which throws off all calculations based off of load and causes you to be a slave to doing frequent battery draindowns and idle relearn procedures..........

Right now when you look at a stock cam vs the stage 1 camshaft the overlap between the two its evident that with a stock injector timing table youre puddling up fuel inside your plenum right now and if you remove the upper intake will probably find its all wet.



My injector timing settings are all changed from the stock setting above and I had to raise my highslope in the injector setting by 5 points just to smooth out the injector pulsewidth with the 255lph fuelpump in the fueltank and the reason why I run the 255 is because I have a water seperator filter aswell as a couple super microfine fuel filters one for the engine and the other for the N20 and never want to guess an issue as being fuel related especially when Im running very safe....LOL






You'll also want to run in Open Loop.....

I just went through a similar issue except I have the tuning chip and couldnt get idling smoothed out enough without leaning it out then when the adaptives kicked after entering closed loop so I had to force it into open loop and then get the fueling worked out manually.

Heres a logfile from a direct startup then doing the bandaid way of upping the timing and t-stop screw just to see what happens......







In the final stages of my tuning was to slowly back down the TB throttlestop screw until I got the idle I wanted as the ratch followed no problem..

I couldve backed out the throttlestop screw a tad more to put the IAC to more work but I like when the rpm's dont dip down alot better as it gives crisp flawless shifts with a manual///.......This below was one of the logs that was in phase with setting idle and AIC/ISC......




I went from a 1/16" stock pin hole in each throttlebody flap to slightly larger 1/8" holes in both flaps and changed the injector timing table to match the camshaft events of my camshaft plus turning on closedloop to get the idle smoother than I ever imagined..



In my old 89 Mustang w/347 w/vortec I had to drill the throttlebody flap hole to 7/32".



Your ECU has a backflowclip setting allowance for a stock engine...On a modified engine you want to adjust this so its not an issue...





All values on the lefthand column is my tune and the ones on the right highlighted are the stock settings....


My new idle settings for my camshaft recommendations are custom set in the ECU to 800rpms with all other parameters changed and raised accordingly so proper calculations can takeplace alot faster..

When you raise the idle just by turning the throttlestop screw is all wrong as theres many parameters that must match the new idle needs in order for the ECU to calculate and function correctly and make your engine run as efficiently as possible...







Seeings 800rpms is 128 rpms over stock setting everything in alot of parameters must be raised by that same 128 or calculations will be way off at WOT...






The other things needed to change was rpm gain multiplier in ISC which I changed to all 2's and it stops the wicked idle swings and surging....youll also need to lower the IAC opening in the ISC transfer section asell as adjust the a few other parameters to get your A9l to adjust correctly..

That big checkmark doesnt work very well with aftermarket camshafts.........LOL




I also did some rescaling of the parameters too to custom suit the modified non-stock combo...








Im running a 70mm MAF Cardone #74-9518 from a 94 T-Bird with a twin 35mm TB that came stock on the 351w engine I put in my 89 but slightly modified..

One good thing about that MAF is it doesnt care about the injector size due to the ability to run on vehicles with both 19lb injectors and 36lb injectors and also found out that it has no problem running on 24lb injectors aswell as after you get the fuelling more better you can finetune the MAF airflow which can be done via this way with a method called a MAF autotune..





But like I stated if your fuelling isnt good this wont work because adaptives must be abled and also so does closed loop mode....


Good Luck

Jesus...lots of these cars are running with no chip on a HCI swap with a bigger MAF calibrated for the bigger injectors. Something else is going on here. Drilling a hole in the blade of the throttle body is a band aid for a problem. Tuner did that on my car and I tossed the TB, installed a new one and found the problem. You have to have a closed PCV system with the mass air so no open oil fill caps and no vacuum leaks.

Tuner drilled mine and idle was from day one. Removed it, installed the new one, base idle reset and problem went away. Once you put a hole in it there is no going back. Does it work? I would say yes. How well does it work and is it the right way to do it? I would say no being Ford did not fix them that way that I know of.

There was a recall on them due to a fire hazard. They have issues with the metal part separating from the plastic part and wires getting hot. Most if not all of Fox Mustangs have had the ignition switch replaced once and some a couple of times. Examples of failures:

Melted wires and connector:



Plastic part and metal part separation with zip tie"fix":